Understanding duration, work, and units

- When you first assign resources to tasks, Duration, Work, and Units are connected by a simple formula. But there are a few subtleties you should understand in order to get your resource assignments right every time. First, let's make sure we understand what these fields represent. Duration is the length of a task in work time, from its start to its finish. If a task starts first thing Monday morning, and ends on Friday afternoon, the Duration is five work days.

Work represents the person hours it takes to complete a task. A task might have a Duration of five days. If you're going to work 10 hours on the task, the Work is 10 hours, regardless what the Duration is. Assignment Units are the percentage of the assigned resources' available time dedicated to the task. In Project's task form, the label just says "Units". If resources are fully dedicated to a task, units are 100%. If resources work half their available time, units are 50%.

Basic algebra is all it takes to understand the relationship between Duration, Work, and Units. Duration equals Work divided by Units. Let's say that Work is 40 hours. The resource is going to work on the task full-time, that is, 100%. Doing the math gives you a Duration of 40 hours, which is the same as five work days. Switching the formula around, Work equals Duration times Units.

If Duration is five days and the resource is going to work 40% on the task, the Work is two full work days, or 16 hours. If you know Work in a Duration, Project calculates units. Say you set the Work at 40 hours, and the Duration at four weeks, which is 160 Work hours. The percentage of time the resource has to work on the task to get it done in that duration is 25%, shown here as 0.25. Project has built-in rules about which value it calculates.

Unless you give Project specific instructions, the program tries to calculate Duration first. That's because Duration is usually the value that has to give. Then it calculates Work, and finally, only as a last resort does it calculate Units. Let's say that you fill in "Work" and "Units". Project calculates "Duration". But what if you fill in only "Work"? Project still calculates "Duration". How does it do that? It sets the "Units" to 100%, or the resource's maximum units.

If you fill in "Duration" and "Units", Project calculates "Work". Once again, if you don't fill in "Units", Project uses 100% or the resource's maximum units so it can calculate "Work". Only if you give it both "Duration" and "Work" does it calculate "Units". That's how Project handles Duration, Work, and Units when you first assign resources to tasks. Another movie explains how Project calculates these fields when you modify resource assignments.